A type of electro magnetic relay is known in the prior art in which, after the relay has received a signal which commands closure of its contacts, a holding voltage must be applied to the relay coil in order to hold the contacts in their closed position for so long as a further command, for opening the contacts, is not received. This type of relay may be provided with a return spring for moving the moving contact of the relay away from the other contact so as to open the contacts. The open position is also sometimes known as the "non-adhered contact position", and in this specification the position of the contacts in which they are firmly engaged together will sometimes be referred to in terms of "adhesion", it being understood that this does not imply actual bonding.
The above mentioned holding voltage is usually of a lower value than the initial voltage which energizes the relay coil so as to cause its moving contact to be brought into engagement with the other contact. For this reason, during the holding phase, the current which is consumed by the relay coil, under the reduced holding voltage, is also smaller; this is because the holding state only requires the provision of enough electrical energy to counterbalance the effect of the return spring of the relay. The relay has only a very small air gap when closed, while, since in the open condition the air gap is greater, in order to close the contacts there is a need for a higher magnetising current.
In some applications, relay boxes are required which call for a battery of relays, some of which may be held simultaneously in their closed (or adhered-contact) position. Due to the high cumulative current consumption in such a battery of relays, and in particular because the resistance of the relay coils is quite low, a large amount of heat is given off. This is the main drawback of these prior art arrangements.
An alternative technology does exist, in which the electromagnetic relays are replaced by semiconductors. However, this technique has the disadvantage of increased cost, while in general terms the control circuits of such semiconductor interruptors are considerably more complicated.